Flash back to April 16. Sparkly Cubs all-everything prospect Kris Bryant had an on-base plus slugging percentage of over 1.000 at Triple-A Iowa, while the Cubs were a not-bad 5-3 and ignoring the clamor to bring up Bryant, waiting until his service clock reset and they could keep him for another year of discounted team control in 2021. Mike Olt and Jonathan Herrera, who played the most games at third in his absence, were hitting .133 and .214, respectively.
Bryant came up on April 17. A month later, they were four games over .500; today they're 14 games over .500, and are 9-1 in their last 10 games. He's their second-best hitter behind Anthony Rizzo, and their third most valuable player behind Rizzo and Jake Arietta. He's clearly made a huge difference, as the Cubs hold a somewhat surprising 3.5 game lead over the defending World Champion Giants—whom they just swept—for the final wild-card spot.
But would the Cubs be better off had Bryant been in the lineup all season? Probably not, but some loose and very generous calculations suggest Bryant might have bought them a tiny bit of breathing room. Obviously a lot of things could have happened, and can still happen, that could chaos-theory the Cubs in or out of the playoffs—but one of those things that did happen was keeping Bryant down on the farm, and playing with the numbers highlights the potential gravity of such a decision.
In April, the Cubs' three non-Bryant third basemen were worth about -0.1 wins above replacement over eight games. In other words, just slightly worse than your typical fill-in, which is essentially what they were.
Bryant, in 12 games, was worth 0.7 wins above replacement, or 0.06 WAR per game, per Fangraphs's numbers.
That's really good, because Bryant started off on fire, hitting .318 in April with a spectacular .455 on-base percentage and playing good defense. Over 162 games, 0.06 WAR per game comes out to 9.7 WAR, or almost as good as Mike Trout's best seasons. Bryant's not that good, at least not yet. (He'll probably finish with around five or six WAR.)
But if you assume he would have played that well over the first eight games of the season, that's an additional 0.5 wins above replacement. If he'd played every game in April and played that well, he'd have put up 1.2 WAR. Factor in his teammates' (lack of) contribution, and that's 1.3 WAR overall, compared to about 0.6 WAR the Cubs got from third base in April. In an alternate universe, maybe those approximations turn into one actual win.
We're counting angels on the head of a pin, but you can imagine a scenario in which that win was on Tuesday, April 14. The Cubs lost to the Reds by one run, and Herrera and Olt went a combined 0-4 for a combined -0.5 win probability. In their three losses before Bryant came up, Olt and Herrera were a combined 1-12 at third base with one total base and one RBI.
They weren't much better when the Cubs won. In the first eight games of the season, the Cubs' third basemen went 3-27 with two RBI and one run. In Bryant's first eight games, he went 16-33 with seven RBI and five runs. In his next eight games, Bryant went 4-34 with three RBI and four runs—quite a falloff, but still better.
Meanwhile, both Fangraphs and Baseball Prospectus project the Cubs to make the wild-card game with four games to spare over the Giants. Both give the Cubs about an 80 percent chance of making the playoffs, with the Giants about 20 percent. They're likely to get in without eight extra games of Bryant, but it's not preposterous to imagine an alternate universe in which his presence might have made the difference.
But that's not all the Cubs had to worry about. Very generously, I've estimated that eight games of Bryant would be worth about half a win over replacement. But missing those games gave the Cubs an additional year of team control. Dave Cameron estimates that the savings the Cubs get from having one year of a likely superstar at a discount is worth $15 million, or one pretty good player, and that the cost of one win is $10 million. So if—even more generously—the Cubs would have actually won one more game with a full season of Bryant, it still wouldn't be worth it. They've been in a win-very-soon mode, not a win-now mode, and keeping Bryant down fits that strategy. And right now they're on pace to win now anyway.